Fiber Optics & Applications

FIBER OPTICS & APPLICATIONS

Introduction of Optical fibers:

In 1952, physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber. Optical fiber is a medium for carrying information from one point to another in the form of light. An optical fiber (or fibre) is a glass or plastic fiber.Unlike the copper form of transmission, fiber optics is not electrical in nature.

The principle of optical fiber is Total Internal Reflection.

Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of communications with out loss of energy. Optical fibers are also used to form sensors, and in a variety of other applications.

Structure of Optical fibers:

An optical fiber is a cylindrical dielectricwaveguide that transmits light along its axis, by The principle of optical fiber is Total Internal Reflection.. The fiber consists of a core surrounded by a cladding layer.

Cross section of an Optical fiber

The core and cladding are manufactured together as a single piece of silica glass with slightly different compositions, and cannot be separated from one another. The refractive indexof the core must be greater than that of the cladding. The glass does not have a hole in the core, but is completely solid throughout. The third section of an optical fiber is the outer protective coating. This coating is typically an ultraviolet (UV) light-cured acrylate applied during the manufacturing process to provide physical and environmental protection for the fiber.

The core is the central region of an optical fiber through which light is transmitted. In general, the telecommunications industry uses sizes from 8 micrometers (µm) to 62.5 micrometers. The diameter of the cladding surrounding each of these cores is 125 µm. The coating size can vary, but the standard sizes are 250 µm or 900 µm.

Principle:

The Principle of optical fiber communication is Total Internal Reflection.

Total Internal Reflection:

When a light ray traveling in one material hits a different material and reflects back into the original material without any loss of light, total internal reflection occurs.

Fig-1

Since the core and cladding are constructed from different compositions of glass, theoretically, light entering the core is confined to the boundaries of the core because it reflects back whenever it hits the cladding. For total internal reflection to occur, the index of refraction of the core must be higher than that of the cladding.

Let us consider a light ray “A” traveling in a medium of refractive index n1 (core) is incident on another medium of refractive index n2(cladding) where n1 > n2 (fig 2).

Fig-2

When a light ray travels from a denser medium to rare medium, the ray deviates away from the normal. Let the incident ray A makes an angle θ 1 with the normal in the medium of refractive index n1. This ray is refracted into the medium of refractive index n2 and this ray travels away from the normal since

n1 > n2.

When the angle of incidence θ1 is increased and when it becomes θ2 called the critical angle, the reflected ray just emerges along the boundary of separation along OX and here the angle θ2 = 900 (fig 2).

When the angle of incidence is greater than the critical angle θc , the incident ray is reflected back into the same medium (fig 2). This phenomenon is known as total internal reflection.

Relationship between θ1 and θ 2 :

According to Snell’s law, the relationship between the angle of incidence θ1 and the angle of refraction θ2 is given as Fig 2 .

Sinθ1/Sinθ2 = n2/n1

or,n1 Sinθ1 = n2 Sinθ2

n1 Sinθc = n2 Sin 900( θ1 =θc ; θ2= 900)

or,Sinθc= n2/n1

θc= Sin -1 (n2/n1)

The Conditions for total internal reflection:

1. The ray must pass from denser medium to rarer medium.
2. The angle of incidence must be greater than the critical angle (θc).

Propagation of Light through Optical fibers :

Let us consider a ray of light enters from air into the core at an angle θ0 to the fiber axis. The ray is refracted along OB at an angle θr in the core. It further proceeds to fall at the critical angle of incidence equal to (θ1= 900 – θr) on the interface between core and cladding. Since the angle of incidence at the core cladding surface is the critical angle, the ray is refracted at 900 ie, it moves along BC.

Any ray which enters into the core at an angle of incidence less than θ0 will have refractive index less than θr . Due to this, the angle of incidence at the core cladding boundary will be more than critical angle of incidence. Thus the ray will be totally reflected.

However, any light ray that enters at an angle of incidence greater than θ0 at 0 will ultimately be incident at the core cladding boundary will de less than critical angle. Due to this, it will be refracted into the cladding region and be absorbed by the cladding and hence it will be lost. Now if OA is rotated around the fiber axis keeping θ0 be the same, then it describes a conical surface. This cone is referred to as a Acceptance Cone.

Now appling Snell’s law, at the point of entry of the ray A, we have

n0sinθ0= n1sinθr(1)

At the point B on the core cladding boundary, angle of incidence

θ1 = 900 – θr.

Then again appling Snell’s law at the point B on the core cladding boundary,

=

(2)

From(1),

=

Substituting for cos θr, we get

=

Acceptance angle,

θ0= sin-1[]

for air medium, n0=1.

θ0= sin-1

Thus the max angle at (or) below which the light can suffer Total Internal Reflection is called Acceptance Angle. The Cone is referred as Acceptance Cone.

Fractional Refractive Index change:

The fractional differencebetween the refractive indices of the core and the cladding is known as fractional refractive index change. It is expressed as,

∆ = (n1-n2)/ n1

This parameter is always positive because n1 must be larger thann2 for the total internal reflection condition. In order to guide light rays effectively through a fiber, ∆<1. Typically, it is the order of 0.01.

Numerical Aperture:

The numerical Aperture (NA) is defined as the Sine of the acceptance angle. Thus,

NA= (1)

=

=

Approximating n1+ n2 ≈ n1. we can express the above relation as,

= 2 n12 ∆

NA = n1 √2 ∆(2)

Numerical aperture determines the light gathering ability of the fiber. It is a measure of the amount of light that can be accepted by a fiber. It is seen from equ(1) that NA is dependent only on the refractive indices of the core and cladding materials. Its value range from 0.13 to 0.50. A large NA implies that a fiber will accept large of light from the source.

NA increases with increase in acceptance angle(θ0).

TYPES OF OPTICAL FIBERS

Optical fibers are classified into 3 major categories based on

(i)Material used

(ii)Number of modes and

(iii)Refractive index profile

GLASS AND PLASTIC FIBERS

Based on materials in which the fibers are made it is classified into two types as follows:

Glass fibers:

Such fibers are made up of mixture of metal oxides and silica glasses.

Examples: the glass fibers can be made by any one of the following combinations of core and cladding.

(i)Core: SiO2 ; Cladding: P2O3 – Si O2

(ii)Core: Ge O2-Si O2;Cladding: Si O2

Note: If P2O3 is added with SiO2, the refractive index of SiO2 may be decreased. Similarly if GeO2 is added with SiO2, the refractive index of SiO2may be increased.

Plastic fibers:

If the fibers are made up of plastics which can be handled without any care due to its toughness and durability it is called plastic fiber.

Examples: The plastic fibers are made by any one of the following combinations of core and cladding.

(i) Core: Polymethyl methacrylate;

Cladding: Copolymer

(ii) Core: Polystyrene

Cladding: Methyl methacrylate

SINGLE AND MULTIMODE FIBERS:

Mode is the one which describe the nature of propagation of electromagnetic waves in a wave guide (Mathematical concept)

Based on the modes of propagation the fibers are classified into two types, Viz.,

1. Single Mode fibers
2. Multi Mode Fibers

1. Single Mode Fibers

In general the single mode fibers are step index fibers. These type of fibers are made from doped silica. It has a very small core diameter so that it can allow only one mode of propagation and hence called single mode fibers. The cladding diameter must be very large compared to the core diameter. This in the case of a single mode fiber, the optical loss is very much reduced. The structure of a single mode fiber is as shown in Fig .

Structure

Core Diameter: 5 – 10 µm

Cladding Diameter: Generally around 125 µm

Protective Layer: 250 to 1000 µm

Numerical Aperture: 0.08 to 0.10

Band width: More than 50 MHz km

Application

Because of its high band width, they are used in long haul communication systems.

2) Multi-Mode fibers

The multi mode fibers are useful in manufacturing both for the step index and graded index fibers. The multimode fibers are made by multi-component glass compounds such as Glass-clad Glass, Silica-clad Silica, doped silica etc., Hence the core diameter is very large compared to single mode fibers, so that it can allow many modes to propagate through it and hence called multi-mode fibers. The cladding diameter is also larger than the diameter of the single mode fibers. The structure of the multimode fiber is as shown in Fig.

Structure

Core Diameter: 50 - 350 µ m

Cladding Diameter: 125 – 500 µ m

Protective layer Diameter: 250 – 1100 µ m

Numerical Aperture: 0.12 – 0.5

Band Width: Less than 50 MHz km

Application

Because of its less band width it is very useful in short haul communication systems.

DIFFERENCE BETWEEN SINGLE AND MULTIMODE FIBER:

No. / SINGLE MODE FIBER / MULTI MODE FIBER
1. / In single mode fiber only one modecan be propagated. / The fiber in this case allows large number of modes for light to pass through it.
2. / The single mode fiber has a smaller core diameter and difference in refractive index of core and cladding is small. / Here, both the core and cladding refractive indices difference is large as the core diameter is large.
3. / No dispersion (ie. There is no degradation of signal during propagation) / Dispersion is more due to degradation of signal due to multimode.
4. / Since the information transmission capacity is inversely proportional to dispersion (T α ) the fiber can carry information to long distances. / Information can be carried to shorter distance only.
5. / Launching of light and connecting two fibers are difficult. / Launching of light and also connecting two fibers is easy.
6. / Installation (fabrication) is difficult as it is more costly. / Fabrication is easy and the installation coast is low.

STEP INDEX AND GRADED INDEX (GRIN) FIBERS:

Based on the variation in the refractive index of the core and the cladding, the fibers are classified into two types, viz.,

1. Step index fiber and
2. Graded index fiber

1. Step index Fiber

Here the refractive indices of air, cladding and core varies by step by step and hence it is called step index fiber.

In step index fiber we have both single mode and multi-mode fibers as shown in Fig 1 and Fig 2 respectively.

In both the fibers the variation in refractive indices will be in step by step. Since a single mode fiber has less dispersion than multimode, the single mode step index fiber also has low intermodal dispersion compared to multimode step index fiber.

Fig-1

Fig-2

2) Graded Index Fiber (GRIN Fiber)

Here the refractive index of the core varies radially from the axis of the fiber. The refractive index of the core is maximum along the fiber axis and it gradually decreases. Thus it is called graded index fiber. Here the refractive index becomes minimum at the core-cladding interface.

In general the graded index fibers will be of multimode system. The multimode graded index fiber has very less intermodal dispersion compared to multimode step index fiber. A typical multimode graded index fiber is as shown in Fig- 3.

Fig-3

FABRICATION METHODS:

Fabrication of all glass fibers is a two stage process. The first stage consists of producing a pure glass and converting it into a rod or performs. In the second stage, a pulling technique is employed to make fibers of required diameters. Various methods are in use for producing pure glass for optical fibers. These may be grouped into two categories (i) liquid-phase (or melting) methods and (ii) vapour –phase oxidation method.

But here the double –crucible method for fabricating fiber is described. This method comes under the melting methods.

Double Crucible method:

The apparatus for the double crucible method is shown in Fig.

It consists of two concentric platinum crucibles (also called a double crucible) mounted inside a vertical cylindrical muffle furnace whose temperature may be varied from 800º C to 1200º C. The starting material core and cladding glasses, either directly in the powdered form or in the form of preformed rods– is fed into the two crucibles separately. Both the crucibles have nozzles at their bases from which a clad fiber may be drawn from the melt in a manner similar to that shown in the Fig. Index grading may be achieved by diffusion of dopant ions across the core-cladding interface, within the melt. Relatively inexpensive fibers of large core diameters and therefore, large numerical apertures may be produced continuously by this method. An attenuation level of the order of 3 dB/km for sodium borosilicate glass fiber, which is prepared using this technique.

FIBER SPLICES:

A fiber splice is a permanent joint formed between two optical fibers. Splicing is required (i) when the length of the system span is more than the manufactured cable length and (ii) when the cable is broken and needs to be repaired. The primary objective of splicing is to establish transmission continuity in the fiber-optic link. This can be done in two ways, namely, through (i) fusion splices (ii) mechanical splices.

In order to achieve a low-loss splice, it is essential for the fiber ends (to be joined) to be smooth, flat, and perpendicular to the core axes. This is normally achieved using a cleaving tool (a blade of hard metal or diamond). The technique is called ‘scribe and break’ or ‘score and break’.

It involves scoring the fiber under tension with a cleaving tool, as shown in Fig 1. This generates a crack in the fiber surface that propagates in the transverse direction and a flat fiber end is produced.

Fig 1.

Fusion Splices:

A good quality permanent joint may be obtained by fusion or welding the prepared fiber ends. A widely used heating source for fusion is the electric arc. The set-up for arc fusion is shown in Fig 2. Herein, the prepared fiber ends are placed in a precision alignment jig. The alignment is done with the help of an inspection microscope (not shown). After the initial setting, a short arc discharge is applied to ‘fire polish’ the fiber ends. This removes any defects due to imperfect cleaving. In the final step, the two ends are pressed together and fused with a strong arc, thus producing a fusion splice. A possible drawback of such a splicing mechanism is that the heat produced by the welding arc may weaken the fiber in the vicinity of the splice.

Fig-2

Mechanical Splices:

There are several mechanical techniques of splicing fibers. These normally use appropriate fixtures for aligning the fibers and holding them together. A popular technique, known as the snug tube splice, uses a glass or ceramic capillary with an inner diameter just large enough to accommodate the optical fibers, as shown in Fig-3.

Fig-3

The prepared fiber ends are gently inserted into the capillary and a transparent adhesive (e.g epoxy resin) is injected through a transverse hole. The adhesive ensures both mechanical bonding and index-matching. A stable low-loss splice may be obtained in this way but it poses stringent limits on the capillary diameters.

A slightly different technique uses an oversized metallic capillary of squae cross section, as shown in Fig -4.

Fig -4

The capillary is first filled with the transparent adhesive, after which the prepared fiber ends are inserted into it. The two fiber ends are forced against one of the four inner corners of the capillary.

Other techniques of mechanical splicing normally employ V-grooves for securing optical fibers. The simplest technique uses an open V-groove, into which the prepared fiber ends are placed as shown in Fig 5. The splice is accomplished with the aid of epoxy resin.

Fig -5

It is also possible to obtain a suitable groove by placing two precision pins (of appropriate diameter) close to each other. The fibers may then be placed in the cusp as shown in Fig- 6.

Fig -6

A transparent adhesive ensures bonding as well as index-matching, and a flat spring on the top applies pressure ensuring that fibers remain in their position. Such a groove is called as spring groove.

There is yet another technique that utilizes the V-groove principle to realize what is known as an elastomeric splice, shown in Fig 7. In this method, the prepared fiber ends are sandwiched between two elastomeric internal parts, one of which contains a V-groove. An outer sleeve holds these two parts compresses so that the fibers are held tightly in alignment. Index-matching gel is employed to improve its performance. Originally, the technique was developed for coupling multimode fibers, but it can also be used for single-mode fibers as well as fibers with different core diameters.

Splicing with most of these techniques, if properly carried out, results in splice loss of about 0.1 dB for multimode fibers. Some of these can also be used for splicing single-mode fibes.

Fig 7

Multiple Splices:

For ribbon cables containing linear arrays of fibers, the following technique has been used. In this method, shown in Fig 8, the fiber ends are individually prepared, and then placed in a grooved substrate. Adhesive is then used for bonding and index-matching. A cover plate retains the fibers in their position and also maintains mechanical stability.